How can I convert a hair-particle system into 3D, editable objects?
In the modifier panel click the convert button.

 

That should do the trick.

Two things to note:


If you're using an object, the objects will be linked when converted.
The original emitter will continue to emit particles. Make sure to either delete the emitter, or delete the particle system.


By the way, CGCookie has a tutorial (archive) on creating a feather if you're interested.